The present invention relates to a process for fast warm-up of piezoelectric crystal plates and an apparatus for practicing the process. More specifically, the invention relates to a fast warm-up, oven controlled oscillator assembly.
It is well known that the resonant frequency of crystal resonators varies with temperature. The magnitude of this variation depends upon a number of factors, including the type of crystal used, and the angles of cut of the crystal. Since most applications of crystal oscillators require stable frequencies, it is necessary to control or compensate for the temperature effects.
One method of minimizing the temperature effects employs temperature compensated crystal oscillators. As is well known in the art, such oscillators minimize the temperature effects by means of external circuitry, such as the combination of a thermistor network and one or more varactors. Such circuitry can compensate for most of a crystal's frequency changes with temperature.
The highest possible stability is achieved by operating the crystal in an oven controlled oscillator. In such an oscillator, the temperature of the crystal is maintained constant in an oven. The temperature of the oven is set to a "turnover point", i.e., a point where the crystal's frequency versus temperature curve has zero slope.
A number of types of such oven controlled oscillators are known. Generally, the crystals in the ovens are heated by conduction. For example, Ho et al., Proceedings of the 31st Annual Frequency Control Symposium, pages 421-428, (1977), discuss the thermal design of an oscillator using a double oven in which a booster oven in addition to a conventional dual oven is used for fast repeatable warm-up characteristics. The oscillator and crystal are said to be enclosed in the ovens which in turn are mounted within a Dewar flask. The arrangement is illustrated on page 423 in FIG. 2 of the article. The warm-up characteristics of the arrangement at 25.degree. C. are said to be 5.times.10.sup.-9 at 6.8 minutes after turn-on.
Also, in the same volume of Proceedings of the 31st Annual Frequency Control Symposium, pages 3-16, (1977), Kusters et al. discuss the use of a thermal transient compensated crystal cut in combination with a fast warm-up oven which is said to reach its operating temperature in 300 seconds. In addition, Babbit in Proceedings of the 31st Annual Frequency Control Symposium, pages 412-420, (1977), discusses an oven for piezoelectric crystals which is a single oven with rigid closed-cell freon-filled urethane foam insulation, which again heats the crystal blank by conduction. FIG. 4 on page 417 of the Babbit article illustrates the oven details.
Tinta et al. in Proceedings of the 24th Annual Frequency Control Symposium, pages 157-163 (1970), disclose contact heaters for crystal plates. The contact heaters consist of thin metallic films deposited on a crystal surface which are traversed by electric currents. Such contact heaters were said to require very short times for temperature stabilization; however, the experiments reported did not result in any commercial products. The approach was abandoned, probably due to the fact that the crystal blanks had to be heated in an anisotropic manner, which resulted in undesirable thermal transient effects.
The above-discussed methods for heating crystal resonators are subject to a number of disadvantages. As discussed above, conduction has been the mode of heating used to date. Thus, the enclosure containing the crystal is usually back-filled with an inert gas, such as helium or nitrogen, to operate as the heat conducting medium. Conduction is a relatively slow process, and warm-up for such ovens usually takes a relatively long period of time, e.g., 5 to 20 minutes. Since the inert gas often contains contaminants, problems with stability can occur. It would be preferable to be able to seal the crystal plate in a vacuum to get maximum stability and good insulation. However, because such ovens operate by conduction, the warm-up characteristics of vacuum sealed crystals are not very good. Moreover, a contact heater on the surface of the crystal plate produces undesirable thermal transient effects.